An emission control system may be a diesel aftertreatment system which includes a selective catalytic reduction (SCR) catalyst and a diesel particulate filter (DPF). When a DPF is used, thermal regeneration may be employed to clean the filter by increasing the temperature and burning soot that has collected in the filter. As the temperature of the DPF increases, the temperature of the SCR catalyst may also increase. Ammonia that is used as a reductant in the catalyst may be desorbed from the SCR catalyst when the temperature increases resulting in ammonia slip from the catalyst. The slipped ammonia may exit the tailpipe and enter the atmosphere and/or the ammonia may be oxidized when passing through the DPF to form NOx, thus increasing nitrogen oxide (NOx) emissions.
One approach to reduce ammonia slip during regeneration is disclosed in US Patent Application Publication 2007/0144152. In the cited reference, a supply of reducing agent is reduced before and during thermal regeneration. By reducing the amount of reducing agent, stored ammonia may be consumed before it is desorbed due to the increased temperature necessary for regeneration and, thus, ammonia slip may be reduced.
Consumption of the stored ammonia may happen slowly as the ammonia is consumed when it reacts with NOx in the exhaust stream flowing through the catalyst. Engine operating conditions, such as an amount of exhaust gas recirculation entering the combustion chambers, may impact (e.g., increase or decrease) the amount of NOx in the exhaust gas varying the time for ammonia consumption and possibly delaying the regeneration of the DPF which may result in an increase of particulate emissions.
The inventors herein have recognized the above problems and have devised various approaches to at least partially address them. Thus, a method for controlling a diesel aftertreatment system coupled to an exhaust system of an engine, the diesel aftertreatment system including an SCR catalyst and a diesel particulate filter, is disclosed. The method comprises, before regeneration of the diesel particulate filter, adjusting an operating parameter to decrease an amount of ammonia stored in the SCR catalyst to a desired amount of stored ammonia, where the desired amount of stored ammonia is varied based on operating conditions, and initiating regeneration of the catalyst when the desired amount of stored ammonia is reached.
Specifically, in one example, the overall amount of NOx emission, and thus the desired amount of ammonia stored in the catalyst, may be reduced before regeneration and may further be based on an amount of NOx generated to reduce ammonia while the catalyst is increasing in temperature. In this manner, heating for regeneration of the DPF may be controlled based on the amount of stored ammonia so that there is a reduced amount of ammonia slip from the catalyst as well as a reduced amount of NOx emission from the tailpipe during thermal regeneration of the catalyst.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.